Humans have been modifying the genetic constitution (genomes) of crop plants for thousands of years, since the very beginning of agriculture. In the past, modifying the genomes of crop plants was accomplished by selecting seeds from those individual plants that produced the most grain, were most resistant to diseases, or were most tolerant of environmental stresses (e.g., drought). These seeds were then used to plant the next year's crop. This approach is sometimes referred to as classical plant breeding, and it has been extraordinarily successful at producing improved crops. However, this approach is subject to a major limitation in that it only allows for the selection of genes (and the associated genetic traits) that are already present in the genome of the crop plant. Although many potentially useful genes are present in the genomes of other organisms, they are not always present in the genomes...